In a statistical model, PTSD mediated the association between exposure at the site and respiratory symptoms among both police officers and other types of responders, Evelyn Bromet of Stony Brook University in New York and colleagues reported online in the journal Psychological Medicine.

The results suggest "an indirect association of exposure with respiratory symptoms through PTSD, a finding that mirrors research conducted with Vietnam veterans," the researchers wrote.

"Mental and physical health are integrally linked," Bromet said in a statement. "It is not always obvious which one is the driver, but, in the end, what matters is that both mental and physical health are recognized and treated with equal care and respect."

So Bromet and her colleagues assessed 8,508 police officers and 12,333 other types of responders who were evaluated at the World Trade Center Medical Monitoring and Treatment Program between July 16, 2002, and Sept. 11, 2008.

They used structural equation modeling (SEM) to explore patterns of association between exposures and other risk factors.

Overall, fewer police than other responders had probable PTSD (5.9 percent versus 23 percent) and respiratory symptoms (22.5 percent versus 28.4 percent), although pulmonary function was similar between the two groups.

They found that PTSD and respiratory symptoms were moderately correlated for both groups.

Exposure was more strongly associated with respiratory symptoms and showed less of an association with probable PTSD. Exposure was only weakly associated with lung function, they reported.

It may be that PTSD, which is associated with immunologic function, can increase pulmonary inflammation, resulting in respiratory abnormalities, or the cognitive processes associated with the condition can increase the perception of respiratory symptoms, they wrote.

On the other hand, chronic respiratory symptoms could serve as reminders of traumatic events and increase PTSD rates, they wrote.

Further longitudinal studies are needed to disentangle the possibilities, they said.

Still, they concluded that "regardless of which came first, PTSD or respiratory symptoms, our findings emphasize that mental health screening is as essential as screening for respiratory symptoms."

The study was limited because the sample was comprised of volunteers, which could introduce bias, and by the fact that police officers may under-report PTSD symptoms because of concern about keeping their jobs.